[MacPorts] #68606: smlnj: update to the new version, retain current as smlnj-legacy (following upstream)

[MacPorts]

#68606: smlnj: update to the new version, retain current as smlnj-legacy (following
upstream)
----------------------------+--------------------------
  Reporter:  barracuda156   |      Owner:  tobypeterson
      Type:  defect         |     Status:  assigned
  Priority:  Normal         |  Milestone:
 Component:  ports          |    Version:  2.8.1
Resolution:                 |   Keywords:  arm64
      Port:  hamlet, smlnj  |
----------------------------+--------------------------

Comment (by ryandesign):

 I don't recommend creating ports whose names include the ambiguous word
 "legacy". If a port is needed for an older version, just use the version
 or branch in the port name.

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Ticket URL: <https://trac.macports.org/ticket/68606#comment:6>
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